Update the index

README.md

@@ -16,6 +16,36 @@ these sources do not share one.
 | FIGARO | [`OptimalSolution/databank-figaro`](https://huggingface.co/datasets/OptimalSolution/databank-figaro) | CC-BY-4.0 | 27 datasets | 862 MB |
 | Natural Earth | [`OptimalSolution/databank-basemaps`](https://huggingface.co/datasets/OptimalSolution/databank-basemaps) | public-domain | 2 layers, 4 resolutions | 41 MB |
 
-`index.json` is the machine-readable catalogue: every source,
-dataset and layer with its licence, coverage, size and manifest
-hash. It is generated from the manifests, never hand-edited.
+## Fetching, not browsing
+
+These repos are built to be QUERIED, not downloaded. The files are
+parquet and Hugging Face serves HTTP range requests, so a client reads
+the footer, picks the row groups it needs and pulls only those column
+chunks. Measured on an 11.8 MB partition of a 165.9M-row table:
+**0.85 MB fetched, 7% of the file**, for a real slice.
+
+`index.json` is what makes that possible without a directory listing.
+It carries every source, dataset and layer with its licence, coverage,
+size, column list, partition key, manifest hash and **the relative
+path of every file** -- so a URL can be constructed rather than
+discovered. It is generated from the manifests and never hand-edited.
+
+```r
+library(duckdb); library(jsonlite)
+idx <- fromJSON(paste0(hub, "/resolve/main/index.json"),
+                simplifyVector = FALSE)
+# pick a source and dataset, then one entry from its `files` list
+con <- dbConnect(duckdb())
+dbExecute(con, "INSTALL httpfs; LOAD httpfs;")
+dbGetQuery(con, sql)   # read_parquet(url) over HTTP, no download
+```
+
+```python
+import duckdb
+duckdb.sql("INSTALL httpfs; LOAD httpfs;")
+duckdb.sql(f"SELECT industry, sum(value) FROM '{url}' GROUP BY 1").df()
+```
+
+The dataset viewer works too, but it is a preview: splits are named
+`train` because that is Hugging Face's default for a dataset with no
+train/test division, not because any of this is training data.